The present thrust in wafer manufacturing is to shrink line widths and spacing to micron or submicron dimensions. It has therefore become increasingly important to provide precise line dimensions or line stability on wafers. While developments in this area are applicable to a wide range of devices, they are particularly important in the area of magnetic bubble memories. Present magnetic bubble memories typically have over 64,000 bits on a chip of less than 1/4 inch on each side. A basic requirement to increasing the density on these chips is to reduce the line widths and spacings and to enhance precision in line width dimensions.
In magnetic bubble memory fabrication the step requiring the most precision in line dimension is the forming of the permalloy pattern. A presently preferred process for forming the permalloy pattern includes nonselectively depositing a continuous layer of permalloy upon a surface of a magnetic bubble memory wafer, which typically includes a gadolinium gallium garnet substrate with layers of silicon dioxide, an aluminum-copper alloy conductor pattern, and another silicon dioxide layer. A positive photoresist pattern is formed on the permalloy layer, leaving exposed those portions of the permalloy to be removed.
The wafer is then placed directly in an ion etching chamber where the photoresist absorbs the ion energy so that all of the permalloy not covered by the photoresist is removed during the etching operation, but the permalloy under the photoresist is left undamaged.
A problem inherent in this technique, involving direct use of photoresist, is that the ion etching operation heats the wafer and raises its temperature to well over 100.degree. C. if no means are used to cool or remove heat from the wafer during the ion etching operation. Since the photoresist deteriorates at temperatures above 100.degree. C., some method must be used to remove the heat from the wafers during the ion etching process.
Several methods have been used in the past to remove heat from the wafers during ion etching operations. One method is to coat the backs of the wafers with vacuum grease or an indium or gallium based amalgam, and then to glue the wafers down to a water cooled base plate. Although effective, this technique is time consuming and introduces contaminants into the chamber during the ion etching operation.
Another method has been to provide a holding plate which contains a plurality of counterbored holes into which the wafer is laid, working side down, while being supported on its outer circumference by the ledge of the hole. The working side of the wafer is exposed through the hole to bombardment by ions coming up from an ion source located in the bottom of the sputtering chamber. A piece of crinkled aluminum foil is placed on top of the wafer to provide uniform heat transfer from the wafer; and then a metal slug is placed on top of the aluminum foil. A clamping or loading spring is placed on top of the metal slug so as to protrude above the top of the holding fixture. Finally, the top of the holding fixture is attached to a water cooled plate which is located inside the sputtering chamber, with the spring claim compressed between the cooling plate and the slug. The heat generated then passes from the wafer through the crinkled aluminum and into the metal slug and then out through the spring clamp. A problem with this method is that the spring clamp is a relatively poor heat conductor and thus a high thermal differential occurs between the metal slug and the cold plate. Thus, the wafer temperature typically is not kept low enough to ensure optimum photoresist characteristics.
A third prior method has been to replace the metal slug and the spring clamp with two parallel plates held apart by a plurality of springs. The adjacent faces of these two metal plates are grooved and spiked so that the two plates intermesh with each other to establish a large surface area between the two plates. These two plates and spring assembly are placed over the crinkled aluminum with the springs compressed, but not to the point that the plates touch each other when the holding fixture is mounted to the cold plate. In this method, some heat is transferred by conduction from the wafer into the lower plate, but the heat transfer from the lower plate to the upper plate is primarily by radiation due to the vacuum environment of the chamber. While the two plates are more efficient than the spring clamp, they are still only marginally satisfactory for this application.
Thus, it can be understood that an efficient heat transfer mechanism for effectively transferring heat from a wafer to a cold plate in the vacuum environment of an RF sputter etching chamber is highly desirable.